1. Field of the Invention
Broadly speaking, this invention relates to a phase-lock loop circuit. More particularly, in a preferred embodiment, this invention relates to a phase-lock loop circuit for use at microwave frequencies.
2. Discussion of the Prior Art
The phase-lock loop circuit is widely used in industry. For example, it may be used as an FM demodulator, a carrier recovery circuit for the synchronous demodulation of digitally-encoded signals and for pilot-tone recovery in suppressed-carrier, single-sideband transmission systems, or the like.
The classic phase-lock loop circuit comprises a phase-detector which compares an incoming signal with the output of a voltage-controlled oscillator (VCO). If there is a difference in frequency, and hence phase, between the input signal and the VCO output an error signal is developed which may be used to alter the frequency of the VCO until its output frequency matches the frequency of the input signal, at which time the error signal vanishes and phase-lock is achieved.
A problem arises, however, when an attempt is made to use the classic phase-lock circuit at microwave frequencies. This is so because the time delay around the phase-lock loop, insignificant at lower frequencies, becomes critical and reduces the pull-in range of the PLL circuit to practically zero at microwave frequencies; thus, phase-lock is never achieved. This problem is further compounded if it is desired to use the PLL circuit with digital transmission systems of the type which employ four-phase, DPSK modulation, for example, the 274 Mb/s, 18 GHz microwave system known in the industry as DR-18A. This latter problem is due to the fact that the beat frequency present at the output of the phase detector, ordinarily merely the algebraic difference between the frequency of the input signal and the frequency of the VCO output, becomes, in a four-phase, DPSK system, four times the beat frequency, thus doubly ensuring that phase-lock will never be achieved.
Although certain specialized phase-lock loop circuits, such as the quad-lock detector or the FM discriminator may, in theory, be used in such microwave systems, they do not meet other objectives, such as system noise, and are thus not practical.